FinFET (Fin Field Effect Transistor) technology builds fins across the top of a silicon substrate. Rather than forming transistors on the surface of the silicon substrate (planar FET), FinFET transistors are formed on the fins. The surface of the substrate is still available to be used for other structures which increase the total number of devices. In addition the effective channel width of a FinFET is larger than for a planar FET.
Any silicon semiconductor transistor, whether of the fin type or planar type, experiences current leaking from the current source (S) to the drain (D) terminals. In order to reduce the leakage in fin FET, the substrate is doped beside and underneath the fin. This doping is done by applying thermal diffusion. This drives the dopant beside and beneath the fin but not on and over the fin.